ime 67 years old ,I got up out of my chair and my legs gave way its only happened once ,what could cause this ?

You probably had a sensory failure or a fuse go out. Check the consol or dashboard somewhere. If that isn't the problem you may have to take it to a mechanic and have him take a good look at it.

Hip or knee failure is one common cause.

If you felt a sharp pain at the moment of collapse (that is: *before* you hit the floor), I guess it is probably one of those two.

Asking in the Cars & Transportation section isn't likely to net many good answers. Try moving your question to the HEALTH category. Good luck.